Duplicate addresses happening on its own. Magic... ? I think not.

[Bluedoor01]

I was given a read only spreadsheet with 3 tabs and I deleted 2 tabs. I then opened the 3rd tab and then deleted some un-needed columns. Then I inserted some lines and manually put in the information. I saved it to my desktop and then shared it in Microsoft teams with a co-worker. When they opened it the lines that I manually put in duplicated addresss from lines already in the spreadsheet. I did not do a dulpicate sweep. Did read only have something to do with it? I cant figure out how the dulpicate addresses happened. Auto fill was off. It was a simple last name, first name, address, city , state and zip spreadsheet wtih 160 names.